Ink jet printing is a non-impact method for recording information in response to an electronic signal, such as that generated by a computer. In the printer, the electronic signal produces droplets of ink that are deposited on a substrate such as paper or transparent film. Ink jet printers have found broad commercial acceptance due to their reliability, relatively quiet operation, print quality, and low cost. Thermal or bubble jet drop-on-demand printers have been especially successful and have found broad application as output devices for personal computers in the office and the home.
Both dyes and pigments have been used as ink colorants for ink jet printers. However, dye-based inks have several disadvantages as compared to pigment-based inks. They are usually very water soluble and remain so after drying on the substrate. They are easily redissolved by contact with water and will run when exposed to a water spill. Also, dye images smear on contact with felt pen markers. In addition, they exhibit poor light stability relative to pigments and are known to fade even under conditions of office fluorescent lighting. Thus, dye-based ink jet inks are often unsuitable for use in applications requiring moisture resistance and greater light stability. Pigments, however, provide improved water and smear resistance and improved light stability.
There are three major ink related concerns in the ink jet printing technology. They are (1) print quality, (2) drying rate, and (3) reliability. Print quality is primarily determined by factors such as optical density, edge acuity/feathering, spray and other artifacts, and mainly the resolution of the printer. Drying rate determines the throughput rate and therefore productivity. One of the major deficiency in the current ink devices is their relatively slow printing speed compared to printers using competing technologies. A major obstacle is the ink drying rate because, in a sheet fed printer, the ink must be dry before the succeeding sheet contacts it or the ink will smear.
Drying occurs by both evaporation of the liquid vehicle and ink penetration into the substrate. Evaporation is determined by solvent vapor pressure whereas penetration is determined by interfacial energy between the ink and the substrate and by the porosity of the substrate. For an aqueous ink, the use of penetrants such organic cosolvents and surfactants is known in the art as a means to increase the rate of penetration of inks.
The third major concern with all ink jet printing devices is obstruction of nozzles during operation and between operations. The obstruction or pluggage is often due to evaporation of the vehicle, causing precipitation or crystallization of the solid components of the ink at the air/liquid interface. In pigmented inks, a further cause of obstruction is the flocculation of dispersed pigment due to a change in solvent composition caused by evaporation. Other reliability issues include storage stability of the ink as seen by physical or chemical changes, compatibility with equipment material, and robustness towards variability in firing conditions.
The use of cosolvents including penetrants, pluggage inhibitors and surfactants to formulate an ink with balanced properties to meet the above mentioned requirements as practiced in the art for dye-based inks, however, tends to cause destabilization of the pigment dispersion. Thus, pigments are a useful alternative to dyes provided the pigment dispersions can be made stable to flocculation and settling. Accordingly, a need exists for dispersants that produce adequate pigment dispersion stability in the presence of other ink components and remain stable throughout the life of the ink jet cartridge.